I'm somewhat divided on this one, but I'm leaning toward that yes, it's a useful tag.
First off, I do think it would have been better to bring it up on Meta before editing a relatively large number of questions, bumping them to the top of the front page. That goes in general, not just about centaur. Yes, this might have been brought up in chat, but far from everyone hangs out in chat; by bringing the matter up on Meta and gathering at least some community consensus, there could have been broader community support and ultimately understanding of what was going on. We also wouldn't be having this discussion.
Second, tags should be used for categorizing questions. You say that "It doesn't make much sense to say "I'm an expert in Centaurs", since they don't really exist and are mostly speculative.", but really, that applies to pretty much everything we do here on Worldbuilding. I doubt there are all that many people who are actually experts in shapeshifters or aliens or alternate-history or time-travel or super-powers or dragons or faster-than-light or ... Well, you see what I mean. We have swaths of well established tags about things that simply do not exist in real life or which in real life are highly speculative.
Centaurs are a specific type of mythological creature, and having a more specific way of categorizing questions about them than the generic mythical-creatures does make some sense. We have a reasonable number of questions about centaurs (21 non-deleted questions as of now that contain the word
centaur), and 15 of those have been tagged centaur, which seems like it exceeds a reasonable threshold for when a specific tag can be useful. It's not really that much fewer than, say, shapeshifters (20 questions thus tagged); let alone the (more dubious, in my opinion) chimera-folk (4 questions).
And of course, if we should have it, it should follow the naming convention of other tags. There's plural zombies, weapons, cities, mythical-creatures, alternate-worlds, spaceships, super-powers, ... so why should centaur be the odd one out? (That, however, can be fixed in a few moments by a diamond moderator, and particularly does not warrant re-bumping all those questions.)